Add wheel support to sky-scrollable

This CL plumbs wheel events through Sky again and uses them in sky-scrollable.

R=ojan@chromium.org

Review URL: https://codereview.chromium.org/876853005

examples/example-scrollable.sky

@@ -32,7 +32,7 @@
 <script>
 module.exports = class extends SkyElement {
   created() {
-    this.cities = cities;
+    this.cities = cities.slice(0, 300);
   }
 }.register();
 </script>